Lipid productivity in TALEN-induced starchless mutants of the unicellular green alga Coccomyxa sp. strain Obi

Starchless mutants of several green algae including Chlamydomonas reinhardtii and Scenedesmus obliquus showed improved lipid productivity. In this study, we isolated mutants of the unicellular green alga, Coccomyxa sp. strain Obi, defective in the gene for the large subunit of ATP:α-d-glucose-1-phosphate adenylyltransferase (AGPL) by means of a transcription activator-like effector nuclease (TALEN) as follows. First, two expression cassettes encoding left and right arms of TALEN for AGPL mutagenesis were introduced into strain Obi, and from two independent transformants of strain Obi carrying both expression cassettes, seven derivatives bearing different mutations in AGPL were isolated. When these mutants were grown in 1/2-diluted and 1/3-diluted minimal media, their lipid contents were significantly higher than those of their parental strains. The growth in 1/3-diluted medium was similar between the mutants and their parental strains, while the growth in 1/2-diluted medium was significantly lower for the mutants than for their parental strains. The lipid productivity, which is a crucial factor for commercial production of biofuels, was not statistically different between the parental and mutant strains. We concluded that the suppression of starch biosynthesis to increase lipid productivity in microalgae is a risky strategy as it could result in low biomass productivity.